Dear Student,
When we say positive or negative force, it is to represent the direction. It is just a convention. You can define forces in one direction to be positive and than the forces, velocity in the opposite direction will be negative.
While walking we exert force backward. Let us assume the direction of walk is positive. In this situation, force exerted by us will be negative and force by friction is positive.
But if we consider the direction of walk to be negative. The force applied by us will be positive and the friction will be negative.
Regards,